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NBSIR 79-1728

A Low-Cost Method for

Measuring Air Infiltration Rates

in a Large Sample of Dwellings

Richard A. Grot

Building Thermal and Service Systems Division

Center for Building Technology

National Engineering Laboratory

National Bureau of Standards

Washington, D.C. 20234

April 1979

Prepared for Community Services Administration *00— 200 1 9th Street, NW

100

Vashington, D.C. 20036

.1156

79-1728

C . 2

Rational Bureau JVN $

NBSIR 79-1728

•M

A LOW-COST METHOD FOR

MEASURING AIR INFILTRATION RATES

IN A LARGE SAMPLE OF DWELLINGS

Richard A. Grot

Building Thermal and Service Systems Division

Center for Building Technology

National Engineering Laboratory

National Bureau of Standards

Washington, D C. 20234

April 1979

Prepared for Community Services Administration

1 200 1 9th Street, NW

Washington, D.C. 20036

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Dr. Sidney Harman, Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

if Stanca 1 -. 1379
'
A LOW-COST METHOD FOR MEASURING AIR INFILTRATION

RATES IN A LARGE SAMPLE OF DWELLINGS

Richard A. Grot

A method for collecting air infiltration data in a large sample

of dwellings is presented. The method consists of a tracer-gas dilution

technique employing air sample bags which are analyzed in a central laboratory. The method will be applied to a Community Services Administration optimal weatherization demonstration in approximately 300 dwellings

on 16 sites throughout the United States. The method will yield air

exchange rates under typical heating season condition for each dwelling

in the demonstrations. Preliminary data on air infiltration rates in

low-income housing in Portland, Maine are presented.

Key Words: Air infiltration; air sample bags; energy conservation;

retrofit; sulfur hexafluoride; tracer gas; weatherization.

iii
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1 . INTRODUCTION

The leakage of air into and out of the living space of a dwelling is one of the major components of heat loss of a home. Though recent

advances in instrumentation have made it possible to automatically

measure the air leakage rate of a building, the techniques to obtain

actual air leakage rates are both expensive and difficult. In an attempt to measure the actual air infiltration rates in low-income

housing and to determine the effectiveness of various weatherization

measures on reducing the air leakage in a home, a measurement technique using a tracer gas and air sample bags has been developed.

Preliminary data on air leakage rates from homes in Portland, Maine

have been collected. These measurements are part of an evaluation

of a weatherization project undertaken by the Community Services

Administration.

The Community Services Administration (CSA), in order to assess how it

might help poor people to cope with increasing energy costs, has undertaken a weatherization demonstration project whose major goal is to

provide data for determining the optimal level of weatherization

which should be applied to the residences occupied by the poor in

various climatic zones in the continental United States. The Center

for Building Technology of the National Bureau of Standards (NBS)

has been given the role of providing technical assistance and support

for this demonstration. This role involves selecting demonstration

cities, selecting homes in each city, selecting weatherization options

both for the building envelope and the buildings’ mechanical system,

supervising the collection of the data from the demonstration, developing test procedures for evaluating the effect of weatherization on the

homes, and analyzing the results of the demonstration. The demonstration project is to be undertaken in 16 cities. These are graphically

shown in Figure 1, along with the degree days for each location. At

each site, approximately 20 to 30 homes will be weatherized.

The principal measure of the effectiveness of the weatherization of

each dwelling will be the observed reduction in the utility bills of

the dwelling. However, in order to assess in more detail the savings

from each option and to explain anticipated variations in the amount

of saving accrued among the homes, a series of tests for both the

building envelope and the mechanical system has been developed. The

building envelope tests currently being planned consist of: 1) a thermographic survey of the exterior of the dwellings, 2) air infiltration tests, 3) determination of the interior temperature stratification, and 4) the determination of the existence of heat by-pass mechanisms into attic and basement. The infiltration tests will consist

of: 1) a test method for determining the actual air leakage rates

in each dwelling, 2) a fan pressurization test, and 3) a test for

locating the air leakage paths still existing in the dwelling after

weatherization, employing fan depressurization and an infrared scanner. The topic of this paper is to describe the test method for determining

the air leakage rate in each dwelling.

1
TACOMA

(5:

851 2

Figure

1.

Site

selected

for

demonstration.
In attempting to assess the air leakage charcteristics of a dwelling,

in the author's opinion, four classes of questions are usually asked:

1) what are the actual air leakage rates occurring in the structure

under various climatic conditions and usage patterns; 2) how tight is

the building compared with other buildings and with itself after cer- tain measures are applied to the building; 3) where are the leakage

paths in the structure; and 4) what is the severity of each air leakage path. The actual air infiltration rate of a building is usually

determined by the tracer-gas dilution method [1, 2]. The tightness

of a building can be obtained by the tracer-gas method, but the

fan pressurization method is easier to perform [3-5], The location

of air leakage paths can be observed using thermography, either

under normal weather conditions [6] or in conjunction with pressurization of the building [5], It is very difficult to determine

the amount of air leakage attributable to each air leakage path.

Attempts at this level of quantification usually consist of sealing

the observed openings and using either the tracer-gas method or the pressurization method to determine the reduction in air leakage.

As mentioned above, the actual air infiltration rates in dwellings

are usually determined by the tracer-gas technique. In the CSA weatherization demonstration, one would like to know the actual air leakage

rates in each of the demonstration homes, at least after weatherization

has been performed and preferably both before and after weatherization.

Though the tracer-gas technique has the reputation of being difficult

to perform and requires both highly trained personnel and expensive

equipment, it is the author's opinion that this method can be modified

through the use of air sample bags such that it can be applied to a large collection of dwellings by relatively inexperienced persons

(possibly even the building occupant) in an inexpensive manner. The method will provide actual air exchange rates under various

weather conditions which can be used to assess the typical air

leakage performance of the building. The following sections will

describe this method.
2 DESCRIPTION OF TEST PROCEDURE

The normal tracer-gas technique for measuring air infiltration consists of injecting a quantity of tracer gas into a building in such

a manner that the tracer gas is well mixed and then to measure the

rate of decay of this tracer gas using appropriate monitoring equipment. Sulfur hexafluoride (SF^) is becoming extremely popular for

this type of work and this paper will be restricted to that tracer

gas. Normally after assuring that the initial quantity of tracergas is well mixed in the dwelling, concentration measurements are made

at 5- to 15-minute intervals. The air exchange rate is determined by

fitting the best exponential curve to the concentration decay data.

The main reasons that this method is complicated and expensive are:

1) the concentration monitoring equipment is costly; ,2) its use requires highly trained technicians; and 3) the duration of the test

is usually 2 to 4 hours. In principle, the method leads to a detailed

record of air exchange rates during the period of the test; however, an error analysis (see Hunt [1]) shows that for short time durations

small errors in the determination of the concentration lead to significant errors in the determination of the air exchange rate. Also for

the purpose of assessing the effect of air infiltration on the building

heating load, the average air exchange rate over a period of one or two hours is of primary interest. For the CSA weatherization

project, one would like to obtain for each weatherized dwelling this

average air exchange rate under 5 to 10 different weather conditions

which are more or less representative of the varying conditions experienced by the building during the heating season. It would require

at least one trained technician and one SF^ concentration-monitoring

apparatus per city to obtain this quantity of data. The resources for

this, both monetary and personnel, are not available.

The proposed method for obtaining these data for the CSA weatherization

demonstrations consists basically of replacing the determination of the

decay rate of the tracer gas by concentration monitoring equipment

deployed in the building with the analysis in the laboratory of air

sample bags filled at intervals of one to two hours duration. The

testing of the building would be undertaken in a process consisting

of seven steps. The steps are graphically shown in Figure 2.

Step 1: Injection of Tracer Gas (SF^) into the Dwelling

A predetermined quantity of the tracer gas SF^ is initially injected

into the building such that the initial concentration of SF^ is 100

to 150 parts per billion or approximately 10 to 15 mil of SF^ per

1000 m^ of living space. The author has been using 30 graduated

syringes for this injection. These can be prepared before the test

or filled from a bottle of compressed SF^ at the site. The injection

is accomplished by slowly walking along the dwelling injecting gas into

each room in a quantity approximately portional to the volume of the room. The graduation on the syringe greatly aids in this process.

4
STEP 1 INJECTION

STEP 3

FILL SAMPLE

BAG

STEP 5

FILL 2nd SAMPLE BAG

STEP 2 MIXING

V2-I ^

STEP 4 WAIT 1-2 hrs.

STEP 6 SHIP BAGS TO LABORATORY

13 ID ill ill Mr HI III IB ui HI

Ml UI IU Ai u

ill III ft I! Ill

til III ft II fll

III Ui IK III III

111 m

ui m

m lb WASHINGTON, DC

20234

FIELD EQUIPMENT

STEP 7 DETERMINE TRACER CONCENTRATIONS

IN LABORATORY © a sf 6 detector

SYRINGES OF TRACER GAS SF B

HAND SAMPLE PUMP AIR SAMPLE BAGS

|2 per floor)

Figure 2. Outline of procedure.

5
Step 2: Mixing of Tracer Gas in Dwelling

A waiting period of approximately 1/2 hour to 1 hour should then be

allowed for proper mixing of the tracer gas. For a building with a forced air heating system, the fan on the furnace can be turned on to

assist in the mixing; however, the author's experience is that natural

convection currents even in dwellings without an air handling system

will well mix the tracer gas at least on each floor of the building

(if the doors between rooms are open). There does seem to be a tendency for the concentration of the tracer gas to be higher on the

upper floors, probably due to the natural convection currents caused

by rising warm air.

Step 3: Fill Initial Sample Bags

After adequate mixing of the tracer gas an initial air sample bag is

filled for each floor of living space. This is accomplished by walking around the floor filling the bag in the process with a small sample

pump. This will provide an integrated sample. The author has been

using a 10-liter, five-layer air sample bag and either a hand pump

(the type used to fill footballs and basketballs) or a small portable

battery-powered pump. The important criterion is that the air sample

bag must be filled slowly, thus ensuring that an integrated sample be

obtained (the hand pump requires about fifty strokes to fill the bag,

the battery pump approximately five minutes).

Step 4: Decay of Tracer Gas

One then waits one or two hours for the tracer gas to decay. During

this period the occupants of the dwelling undergo their normal

activities and the mechanical system is left in its normal operating

mode.

Step 5: Fill Final Sample Bags

The procedure in Step 3 is repeated obtaining a second sample bag of

air for each floor of the building.

Step 6: Ship Sample Bags to Laboratory

The sample bags are then shipped to a center laboratory (in this case

the National Bureau of Standards) for analysis.

Step 7: Determination of Tracer Gas Concentration

The tracer gas concentrations are determined in the central laboratory using an electron-capture gas chromatograph. The air infiltration rate AI is then determined by the equation

AI = J Jin (C,/Co) (1)

Ax 1 z

6
where C-^ and C ? are the initial and final tracer gas concentrations

respectively, and At is the time interval in hours between filling of

the sample bags. For periods of 1 to 2 hours, equation (1) is an accurate

manner for determining the air exchange (see Ref. [1]). The above steps are graphically shown in Figure 2.

The above-described method is easy to perform and inexpensive, and if

several homes are grouped together can be accomplished at the rate of

about 1 house per hour.

3. RESULTS OF PRELIMINARY TESTING

In order to assess the viability of the method for collecting air

infiltration data, series of tests were run both in Portland, Maine

in the dwellings that are being weatherized ,and in a home in Maryland

near the National Bureau of Standards. The first series of tests in

Portland was done in order to obtain primary air exchange rates and in

order to test the assumption of uniform mixing when the previously

described injection procedure was applied. The concentration of SF^ was measured at several points on each floor of the tested building.

Usually no variations in tracer concentration occurred which would lead

to appreciable errors in the air exchange rate for the floor. As mentioned previously, there was a tendency for the concentration of the

tracer gas to be higher on the upper floors than on the lower floors.

This observation was the main reason for specifying in the test procedure that separate air sample bags be used for each floor of the building. A summary of the air exchange rates for this test is given in

Table 1. The house numbers marked with an asterisk* are buildings which

have already been weatherized. These data showed that these buildings were experiencing relatively high air exchange rates.

In order to test the feasibility of using air sample bags, tests were run in a home in Maryland. The sample bags were first analyzed the day

immediately following the collection of the sample. An air infiltration

rate of 0.34 air exchange per hour was found. After a period of three

weeks the sample bags were reanalyzed and an air infiltration rate of

0.31 was obtained. Any change in concentration in the gas sample was within the errors of the experiment, since no care was taken to calibrate

the instrument at these times.

4. CONCLUSIONS

The preliminary testing of this method has indicated that it is a feasible method for obtaining air infiltration data at a low cost and using

inexperienced personnel. Although the method does not produce a detailed

trace of air exchange rate over a period of one to two hours, it does

give the total air leakage of that period, and this is usually of primary

interest to energy conservation analysis. The technique also produces

integrated air samples for each floor and therefore minimizes the effects

of non-uniform mixing.

7
TABLE 1

SUMMARY OF AIR INFILTRATION RATES - PORTLAND, ME.

House

#

Air Exchange Rate

First Second

Floor Floor

: (hr 1

Third

Floor

) Duration

Test

(hr)

Inside

Temp

(°c)

Outside

Temp

(°C)

Wind

Direction

& Speed

km/hr

House

Volume

(m )

7 0.78 0.98 1.4 21.1 -7.8

N-NW

0-16 311

10* 0.93 2.0 24.4 2.8

N-NW

0-16 224

11 0.95 0.93 2.4 21.6 0.0

N-NW

0-16 161

12* 1.52 1.57 1.6 20.5 -5.6

N-NW

0-16 274

15 1.96 1.99 1.67 1.4 20.0 -0.6

N-NW

0-16 747

18 1.10 1.17 2.5 21.6 0.0

N-NW

0-16 490

19 1.83 1.68 1.97 1.4 21.1 2.8

N-NW

0-16 441

23* 0.99 1.19 2.0 22.2 0.0

N-NW

24-48 498

25* 0.80 0.87 2.5 18.9 0.6

N-NW

0-16 501

26* 1.21 1.578 0.8 20.0 0.0

N-NW

24-46 453

* These homes have been weatherized.

8
The analysis of the air sample bags in the laboratory permits the use

of standard laboratory techniques, such as dilution of the sample if

the concentration of gas is above the saturation concentration of the

detector, and the use of reference concentrations for each range of

sensitivity of the detector—techniques which are difficult to apply

in the field. The major disadvantage of this method is that it is a blind test and there are no checks in the field which can assure the

accuracy to which the instructions are followed. It is planned to modify the automatic air infiltration equipment described in Reference [7]

such that it can automatically handle 40 bags at a time. The possibility of automatically processing the air sample bags is the reason

that air sample bags as large as 10 liters were chosen. The technique

will be applied during the 1978-1979 heating seasons to the CSA weatherization demonstrations and further verification of its accuracy and

reliability will be undertaken.

9
5. REFERENCES

1. C. M. Hunt, "Air Infiltration: A Review of Some Existing Measurement Techniques and Data," Proceedings of ASTM Symposium on Air

Infiltration and Air Change Rate Measurement, March 1978.

2. P. L. Lagus , "Air Infiltration Measurement by the Tracer Dilution

Method," Proceedings of ASTM Symposium on Air Infiltration and

Air Change Rate Measurement, March 1978.

3. G. T. Tamara, "Measurement of Air Leakage Characteristics of House

Enclosures," ASHRAE Transactions, Vol. 81, Part 1, p. 202-211,

1975.

4. S. Strieker, "Measurements of Air-Tightness of Houses," ASHRAE

Transactions, Vol. 81, Part 1, p. 148-167, 1975.

5. J. Kronvall, "Testing of Houses for Air Leakage Using a Pressure

Method," ASHRAE Transactions, Vol. 84, Part 1, p. 72-79, 1978.

6. R. A. Grot, D. T. Harrje, L. C. Johnston, "Application of Thermography for Evaluating Effectiveness of Retrofit Measures," Proceedings of Third Biennial Infrared Information Exchange, AGA Corporation, Secaucus, N.J., p. 103-117, 1977.

7. D. T. Harrje and R. A. Grot, "Automated Air Infiltrations Measurements and Implications for Energy Conservation," Proceedings of

International Conference Energy Use Management, Vol. 1, p. 457-464,

Pergamon Press, N.Y., 1977.

ACKNOWLEDGEMENTS

The author wishes to acknowledge the many fruitful conversations he

has had with David T. Harrje of Princeton University on this topic.

10
NBS-1 1 4 A rev. »-76i

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBSIR 79-1728

2.00/6 Accession No. 3. Recipient's Accession No.

4. TITLE AND SUBTITLE

A LOW-COST METHOD FOR MEASURING AIR INFILTRATION RATES

IN A LARGE SAMPLE OF DWELLINGS

5. Publication Date April 1979

$. Performing Organization Code

7. AUTHORi S) Richard A. Grot

8. Performing Organ. Report No. 742 2500

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

10. Project/Task/Work Unit No.

11. Contract/Grant No.

12. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City, state, ZIP) Community Services Administration

1200 19th Street, NW

Washington, D.C. 20036

13. Type of Report & Period Covered

i4. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

Document describes a computer program; SF-185, FIPS Software Summary, is attached.

16. ABSTRACT (A 200-word or less factual summary of most si ghi ficant information. If document includes a si gni ficant bibliography or

literature survey, mention it here.) A method for collecting air infiltration data in a large sample of dwellings

is presented. The method consists of a tracer gas dilution technique

employing air sample bags which are analyzed in a central laboratory. The

method will be applied to a Community Services Administration optimal

weatherization demonstration in approximately 300 dwellings on 16 sites

throughout the United States. The method will yield air exchange rates

under typical heating season condition for each dwelling in the demonstrations.

Preliminary data on air infiltration rates in low-income housing in

Portland, Maine are presented.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a propr-r name;

separated by semicolons)

Air infiltration; air sample bags; energy conservation; retrofit;

sulfur hexafluoride; tracer gas; weatherization.

18. AVAILABILITY [^} Unlimited

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